Measles virus (MV) is a leading cause of child mortality in developing countries despite the availability of a live attenuated vaccine for over 40 years. Severely immune-compromised people are particularly at risk for MV. MV causes periodic outbreaks all over the world; in the U.S., outbreaks are becoming a constant every year. While natural infection with MV elicits long-lasting immunity, protection wanes in vaccinated persons. The most serious manifestations of MV infection, including encephalitis, occur in people with impaired cellular immunity. MV affects the central nervous system (CNS) in up to half of routine cases; with adequate cellular immunity the infection is eradicated, but individuals with impaired cellular immunity are at a disadvantage, and even the vaccine strain can lead to fatal encephalitis in such persons. In a recent MV outbreak in South Africa several people died of MV CNS infection. We analyzed the viruses from these patients and found that specific intra-host evolution of the MV fusion machinery -- receptor binding protein (H) + fusion protein (F) -- had occurred. A mutation in F of the CNS-adapted viruses allows it to promote fusion with less dependence on interaction of H with the two known MV cellular receptors; this F is activated independently of H or receptor. We hypothesize that in the absence of effective cellular immunity, MV variants bearing fusion machinery that enabled efficient spread in the CNS underwent positive selection. We propose to identify the molecular determinants of CNS invasion by MV in the host, and determine whether these CNS-adapted viruses can spread between individuals. In aim 1 we will identify the genetic and functional differences between the H/F fusion machinery of wild-type (wt) MV and the CNS-adapted viruses, to correlate clinical pattern with specific fusion properties. In aim 2 we will determine whether the H/F alterations in the CNS isolates permit transmission to intact or immune- suppressed hosts via normal routes of infection. The project will lead to a full understanding of the mechanism of MV adaptation to CNS tropism, and to an understanding of the transmissibility of CNS adapted strains and consequent risk to normal and immune-compromised hosts.